Telecommunications systems, cable television systems and data communication networks use optical and or Ethernet networks to rapidly convey large amounts of information between remote points. To ensure high reliability and availability in communications networks, including optical and Ethernet communications networks, protection switching is often used. When implemented, protection switching typically provides a primary or “working” path for a network and a redundant or “protection” path for the network. Accordingly, each path may be monitored, and if a fault is detected on the working path, network traffic may be switched to the protection path. An example of protection switching may be Ethernet Linear Protection Switching (ELPS) as defined by the ITU G.8031 standard.
With protection switching in optical communication networks, an optical signal may be transmitted via two or more optical paths between the same source and destination node. A selector at the destination may include a photodetector for each path to monitor signals received from the two or more paths. Based on such received signals, the selector may select one of the signals to be forwarded to a transponder or receiver at the destination node. For example, the selector may determine, based on the photodetector monitoring, whether one of the paths has experienced a loss of signal or “loss of light.” If a particular path experiences a loss of light, then the selector may, via communication of interrupt signals or other appropriate signals, select another path to forward to the transponder or receiver. Such selection may be referred to as a “protection switch.”
The selector may operate in accordance with a protection switching protocol (e.g., ITU G.8031 or other standard). Each protection switching protocol may include a hierarchy for handling user-initiated and auto-fault initiated protection switching requests. Such hierarchy may be implemented via hardware, software, or a combination thereof.
In certain instances, a working path of a protection switching group may be subject to intermittent protection switching interrupts. Such interrupts may be of the nature that the working path may operate normally for periods of time, but intermittently generate interrupts due to a particular problem associated with a communication network, sometimes even in cases in which a working path has not failed and protection switching is not needed to maintain communication of traffic. For example, among the reasons an intermittent protection switching interrupt may occur include, without limitation, a bent transmission medium (e.g., cable or fiber) within the working path; a cut transmission medium within the working path; incorrect or lose transmission media connections to network elements of the working path; a transmission medium short within the working path; electromagnetic interference in the working path; faulty connectors interfacing transmission media to network elements in the working path; loss of heartbeat messages (e.g., Continuity Check Messages) by network elements of the working path due to congestion, collision and/or other cause; and/or a degraded signal along the transmission path.
In traditional communication networks, such intermittent interrupts may result in frequent toggling between the working path and the protection path, causing periodic traffic loss upon the occurrence of each intermittent fault, and heavy use of system resources (e.g., processing and memory capacity) potentially leading to system instability or crashes.
One traditional solution to the problem of intermittent interrupts is to employ a hold off timer. Upon an initial detection of a fault on a working path, the fault may not be reported in the form of an interrupt, and the hold off timer may be initiated. At expiration of the hold off timer, a system may determine whether the fault still exists. If the fault remains after expiration of the hold off timer, the system may then generate an interrupt, and protection switching may occur. If the fault clears during the duration of the hold off timer, then no interrupt is generated and protection switching does not occur. However, a disadvantage of a hold off timer is that during the duration of the hold off timer, traffic may not flow over either of the working path or the protection path, leading to potential traffic loss during such duration. Thus, in the presence of bona fide working path failures, a hold off timer does not serve to reduce interrupts, but may instead lead to traffic loss.